It is often difficult to fabricate multilayer thin film solid state devices and transducers because the successive layers of different materials have different surface properties. The surface properties determine whether or not the two materials will adhere to one another. If a material forms an oxide coating on its surface upon the exposure to the air (i.e., aluminum), it is the properties of the oxide layer which determines the material's surface properties and if it will adhere to the surface of a different material. In the sputtering of two materials with unlike surface properties, a sharp interface is formed therebetween resulting in a cleavage plane.
In the sputtering of thin films, it is common practice to use an adhesion layer material to improve the adhesion between the deposit material and the substrate. This method works well when it is possible to find an adhesion layer whose surface characteristics are compatible with both the material to be deposited and the substrate. But, in thin film technology, it is often found that the adhesion layer may have surface characteristics similar to either the deposit material or the substrate but not both. This limitation restricts the use of a broad range of materials used as the deposit material or as the adhesion layer.
While two materials may have surface properties that will not allow one to adhere to the other, the bulk properties of the same materials may be very compatible and during the sputtering operation combine with each other forming a graded interface with no cleavage plane and a good crystalline bond therebetween. The term "graded interface", as used in this description, is a blending of the deposit material and the adhesion layer materials as the materials are sputtered onto the substrate thereby forming a strong crystalline bond with the surface of the substrate.
The subject invention takes into consideration materials having compatible bulk characteristics irrespective of the surface properties of the same materials.